Broadcast-frequency antennas are important components of most spacecraft. For example, a communications satellite in geosynchronous orbit receives a broadcast-frequency communications signal up-linked from a ground antenna with an onboard broadcast-frequency antenna, amplifies the received signal, and then down-link transmits the amplified broadcast-frequency signal back to earth using a different onboard broadcast-frequency antenna. Other types of spacecraft also conduct most of their communications with earth stations and with each other using onboard broadcast-frequency antennas.
Each antenna has at least one broadcast-frequency antenna reflector, which operates by reflecting a broadcast-frequency signal either to (for signals being received) or from (for signals being transmitted) a broadcast-frequency transceiver. Each antenna reflector must be functional to reflect high power densities of broadcast-frequency signals, but it must, like other spacecraft components, be as light as possible due to the high cost of lifting loads to orbit. It must also have excellent thermal performance, inasmuch as it is heated both by the solar rays and by energy transfer from the reflected broadcast-frequency beam. Because the antenna reflector is relatively large in size, it is made to be very light in weight on an areal basis.
In one existing approach, the structure of the broadcast-frequency antenna reflector that defines its overall paraboloid or other shape is made of a light-weight composite material. Because such a material does not reflect broadcast-frequency energy well, the reflecting surface is covered with a broadcast-frequency reflective coating. The broadcast-frequency reflective coating is usually made of a multilayer coating having 3-7 layers of vacuum-deposited aluminum (VDA), silicon monoxide, and silicon dioxide. An epoxy undercoat may be applied to the composite material before applying the broadcast-frequency reflective coating to seal the porosity of the composite material and to provide a smooth surface for the deposition of the broadcast-frequency reflective coating. Before coating with the broadcast-frequency reflective coating, the surface of the composite material is abraded if there is no epoxy undercoat or grit blasted if an epoxy undercoat is used, to impart sufficiently low specularity to its surface.
The conventional antenna reflector is functional, but it is difficult and expensive to manufacture due to the difficulty in, and expense associated with, applying the multilayer coating system in a reproducible fashion. The thermal, electrostatic discharge (ESD), and specular properties of the antenna reflector therefore vary from antenna reflector to antenna reflector. At least some, and often all, of the layers of the multilayer coating are applied in a vacuum to a complexly shaped surface, and it is difficult to achieve uniform thin coatings by this approach. The thermal-radiative properties of the conventional multilayer coating are not as good as desired, and the ESD performance is not good for some embodiments. It is difficult to precisely control the layer thicknesses to achieve the proper balance of the properties. Further, it is difficult to achieve the required low-diffuseness, textured surface on the coating, which is required to ensure that the thermal energy of the sun is not focused on the sub-reflector (if a Cassegrain antenna) and other feed components of the broadcast-frequency antenna. If there is too much abrading or grit blasting, the deposited VDA layer is not smooth and continuous, which is required for good broadcast-frequency properties. In short, the fabrication of the conventional broadcast-frequency antenna is expensive, time-consuming, difficult to perform, difficult to reproduce, and in many cases, the resulting properties are only marginally acceptable.
There is a need for an improved approach to the fabrication of broadcast-frequency antenna reflectors and other components for spacecraft and for other applications where the article must be broadcast-frequency reflective, be light in weight, and have the necessary thermal-radiative and ESD properties. The present invention fulfills this need, and further provides related advantages such as few process steps and less complexity in the fabrication process.